1. Field of the Invention
This invention relates generally to an optical system for correcting wavefront phase aberrations of a light beam and, more particularly, to a lateral shearing interferometer wavefront sensor and associated optical system that employs a double-shear/full-aperture approach to correct for branch points in the wavefront of an optical beam that has been subjected to aberrations.
2. Discussion of the Related Art
Certain types of optical transmission systems, such as optical communications systems, imaging systems, etc., transmit a coherent light beam carrying information through a medium, such as air. Because the light beam is coherent, the phase of the beam is substantially constant across the beam wavefront when it is generated. However, conditions in the medium, such as turbulence in the air, typically corrupts the beam by introducing distortions that cause some portions of the wavefront to have a different phase than other portions of the wavefront at any given instant in time. If this wavefront phase aberration was not corrected at the receiver, the light beam could not be effectively focused onto receiving optics, such as a fiber optic cable, and thus a significant intensity of the beam could be lost. Therefore, it is known in the art to correct wavefront aberrations at the receiver of an optical system of this type.
Different systems are known in the art to correct wavefront aberrations in an optical system of the type being discussed herein. Typically, these types of systems employ a wavefront sensor, such as a Hartmann sensor, that measures the phase of individual portions of the beam wavefront. Hartmann sensors typically employ an array of lenslets for dividing the wavefront into a matrix of subapertures. Each of the beams in the subapertures is focused by the lenslets onto one or more detectors forming an array of spots on the detectors. The position of the spots provides a direct indication of the wavefront tilt for each subaperture. These tilts are then used by a wavefront reconstructor to generate a surface representative of the phase relationship of the measured beam wavefront. A deformable mirror is employed to generate a compliment of the surface generated by the reconstructor. The aberated light beam is reflected off of the mirror that provides a corrected beam substantially free of wavefront phase aberations. The deformable mirror typically includes an array of actuators positioned behind the mirror that act to deform the mirror at the desired locations to provide the compliment of the sensed wavefront to correct the phase. This process is performed many times a second depending on the particular application.
In another known system, a lateral shearing interferometer (LSI) wavefront sensor is used in combination with the wavefront reconstructor and deformable mirror to provide branch point corrections in the wavefront of the optical beam. A branch point is a point in the wavefront where the phase has a screw-like dislocation. In order to correct a wavefront with branch points, one-wave steps must be made in the surface of the deformable mirror, referred to as branch cuts. One known example of an LSI wavefront sensor is discussed in U.S. Pat. No. 6,163,381, titled Dual Sensor Atmospheric Correction System, assigned to the assignee of this application and herein incorporated by reference.
LSI wavefront sensors provide a copy of the beam being corrected that is shifted in the x-direction by a distance equal to the spacing between actuators on the deformable mirror. The original beam and the shifted beam are combined to provide an interference pattern depicting the phase difference therebetween. The combined beam is then applied to an array of detectors to measure the interference pattern. The intensity of the light measured by the detectors provides a measure of the tilt of the wavefront in the x-direction, and thus the relative phase relationship of the original beam. The same process is also provided to measure the tilt of the beam wavefront in the y-direction.
An example of an LSI wavefront sensor 10 is shown by schematic diagram in FIG. 1. The LSI wavefront sensor 10 employs a Mach-Zehnder LSI 12 including an input beam splitter 14, an output beam combiner 16, two plane reflectors 18 and 20 and a beam shifter 22. An incident beam 24 is split by the beam splitter 14 into first and second split beams 26 and 28 that propagate along separate beam paths, where the two beam paths are equal in length, and are then combined by the beam combiner 16. The split beam 28 propagates through the beam shifter 22, and is shifted thereby. Therefore, when the first and second beams 26 and 28 reach the beam combiner 16, they are offset relative to each other a predetermined distance as set by the beam shifter 22. The beams 26 and 28 are combined as output beam 30, and the interference pattern created by the combination of the beams 26 and 28 is sensed by a detector 32.
The LSI wavefront sensor 10 is able to determine the tilt in the wavefront of the input beam 24 in this manner. The amount of shift of the beam 28 is typically set as the distance between the actuators on the deformable mirror, and is referred to as a xe2x80x9cunit shearxe2x80x9d. In other words, the distance that the split beam 28 is shifted defines areas in the beam 28 that are one actuator apart as compared to the same area in the beam 26. The adjacent portions of the beams 26 and 28 that are interfered with in the combined beam 30 provide a measure of the phase difference between the interfered portions. The detector 32 detects the phase difference between the beam portions because bright areas in the combined beam 30 are constructive interference areas of the beams 26 and 28 that are in-phase, and dark areas in the combined beam 30 are destructive interference areas of the beams 26 and 28 that are out-of-phase. The LSI wavefront sensor 10 provides lateral shearing in one of either the x-direction or the y-direction. A second LSI wavefront sensor is employed for the other direction. The detected signal by the detector 32 is sent to a wavefront reconstructor that then controls the actuators on the deformable mirror, as discussed above.
Two different unit-shear approaches are known to operate an LSI wavefront sensor in this type of system. These approaches include a unit-shear/full aperture approach and a unit-shear/partial aperture approach. As will be discussed in more detail below, when signal levels are high, the partial aperture configuration works the best, as suggested in U.S. patent application Ser. No. 09/410,011, filed Sep. 30, 1999, titled Improved Lateral Shearing Interferometer System, also assigned to the assignee of this application and herein incorporated by reference. However, because the masking of the beams required by the partial aperture approach decreases the available signal, this approach fails at low optical signal levels where the full aperture approach is still working, although at reduced performance.
FIG. 2 is a one-dimensional schematic diagram showing the problem identified above. This diagram shows a surface 40 of a deformable mirror including a first surface portion 42 and a second surface portion 44, where the surface portions 42 and 44 are about one wavelength of the beam 24 apart and are connected by a sloped portion 46. The sloped portion 46 represents a branch cut. An actuator 52 is shown positioned adjacent the surface portion 44, and an actuator 54 is shown positioned adjacent to the surface portion 42. The actuators 52 and 54 push up on the surface portions 42 and 44, respectively, in response to a control signal to generate a compliment of the wavefront surface as calculated by the wavefront reconstructor. The maximum distance the surface portions 42 and 44 can be apart is the one wavelength of the beam 24. As would be appreciated by those skilled in the art, the deformable mirror would include many actuators positioned relative to many branch cuts on the deformable mirror to correct the complete wavefront in this manner.
As discussed above, the first and second split beams 26 and 28 are spatially offset from one another when they are interfered with. This spatial offset is determined by the distance between the actuators on the deformable mirror, where the wavefront of each beam 26 and 28 can be defined as a combination of separate portions between the actuators. For the unit-shear/full aperture approach, FIG. 2 shows a beam area 60 representative of a portion of the first beam 26, and a beam area 62 representative of a portion of the second beam 28. The beam areas 60 and 62 are portions in the beams 26 and 28 that are aligned and interfered with by the combiner 16. In other words, the beam areas 60 and 62 are the same area in the combined beam 30 that were adjacent to each other by the shifted distance in the first and second beams 26 and 28. When the beams 26 and 28 are combined, the beam areas 60 and 62 will align in a one-to-one relationship.
As is apparent, the beam areas 60 and 62 are contiguous with each other and are positioned so that part of the beam areas 60 and 62 are over the sloped portion 46 between the actuators 52 and 54, as shown. Each beam area 60 and 62 has a dimension (unit shear) in both the x and y direction that is the distance between actuators. This causes the beam areas 60 and 62 to contact each other. The full wavefront of the beams 26 and 28 would include many such beam areas relative to the actuators on the deformable mirror in this manner. This diagram only shows one beam area for each of the beams 26 and 28 to simplify the discussion. Because parts of the beam areas 60 and 62 align with the branch cut or the sloped portion 46, the difference in the relative phase over the complete areas 60 and 62 limits the beam reconstruction performance. In other words, when the areas 60 and 62 are interfered, the result is not the desired result, perfect constructive interference, because of the intermediate areas interfering between the zero, or one-wave, areas. This produces a bogus tilt in the wavefront sensor, which causes the wavefront reconstructor to change the surface away from what it should be, resulting in instability and poor performance.
In order to overcome these limitations in the unit-shear/full aperture approach, it has been suggested in the ""011 application that better wavefront sensor performance can be achieved by limiting the size of the areas 60 and 62. Particularly, in the unit-shear/partial aperture approach, the beams 26 and 28 are masked by the LSI 12 so that only those beam portions directly above the actuators 52 and 54 are interfered. FIG. 2 also shows a beam area 64 of the beam 26 and a beam area 66 of the beam 28 that are those portions of the beams 26 and 28 that are directly interfered with when the beams 26 and 28 are combined. The size of the areas 60 and 62 is reduced to form the areas 64 and 66 so that the areas 64 and 66 are not contiguous. The shear distance is still, however, a unit shear. Because the unmasked beam portions defining the areas 64 and 66 are directly aligned with the actuators 52 and 54, these portions do not extend over the branch cut, the sloped portion 46, and do not suffer from the performance limitations discussed herein.
The unit-shear/partial aperture approach corrects the problem with unit-shear/full aperture approach by blocking the light that is not directly on top of the actuators. This improves the performance dramatically, and is the best solution if there is adequate signal. There are situations, however, where the optical signal level is too low so that the unit-shear/partial aperture approach does not have enough light to operate properly. In other words, because a significant portion of the beams 26 and 28 is masked in this approach, in those cases where the total light is already limited, further reducing the amount of light significantly limits the performance of the receiver.
What is needed is an LSI wavefront sensor that has the advantages of the unit-shear/partial aperture approach, but does not suffer from poor performance at low signal levels. It is therefore an object of the present invention to provide such an LSI wavefront sensor.
In accordance with the teachings of the present invention, a lateral shearing interferometer wavefront sensor system is disclosed that employs a double-shear/full aperture approach to correct for branch points in the wavefront of an optical beam that has been aberrated. The wavefront sensor system includes a lateral shearing interferometer having a beam splitter that splits the optical beam into a first split beam and a second split beam, a beam shifter that shifts the first split beam relative to the second split beam, and a beam combiner that combines the first and second split beams into a combined beam. The interference pattern generated by the combined beam is detected by a detector, where beam portions of the first and second split beams are defined in the interference pattern. A wavefront reconstructor receives signals from the detector and reconstructs the beam wavefront. A deformable mirror is provided having a plurality of actuators, where the actuators deform the mirror to correct the beam wavefront.
The interfered beam portions are a double shear in distance apart, or twice the distance between actuators. In this manner, interfering portions of the split beams do not align with branch cuts between the actuators. The interfering portions of the split beams are defined relative to the actuators such that one set of opposing edges of the beam portions align with adjacent actuators, and the other set of opposing edges of the beam portions are positioned between adjacent actuators. The opposing edges of the beam portions between the actuators is in the shear direction.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limited the scope of the invention.